Associated with reduction in size and weight of electronic devices, secondary batteries with high energy density are desired as power supplies for electronic devices. A secondary battery takes out chemical energy, which the positive electrode active material and the negative electrode active material possess, to the outside as electric energy by means of chemical reaction via an electrolyte. Among such secondary batteries, nonaqueous electrolyte secondary batteries that perform charging and discharging by moving ions between a positive electrode and a negative electrode using a nonaqueous electrolyte are known.
A lithium ion secondary battery, which is one type of the nonaqueous electrolyte secondary batteries, is used in various use applications such as mobile phones and electric vehicles (EV). Characteristics demanded for the lithium ion secondary battery includes achieving high energy density, cycle characteristics, and safe operation in various environments.
For a positive electrode current collector of the lithium ion secondary battery, a metal such as Al that forms a stable passive film on a surface thereof for withstanding corrosion due to an electrolyte is commonly used. For example, when Al is used for the positive electrode current collector, a passive film such as Al2O3 or AlF3 is thought to form on a surface thereof. An Al current collector is unlikely to be corroded since the passive film is formed on a surface thereof, and can maintain current collection function.
In recent years, the lithium ion secondary battery has been demanded to be employable satisfactorily even under a high-voltage usage environment (high-voltage usage is defined in the present application as usage at a voltage equal to or higher than 4.3 V). Under the high-voltage usage environment, the Al current collector can easily corrode gradually even when a passive film is formed on a surface thereof, and elution of Al from the current collector becomes a concern.
In an electrolytic solution of the lithium ion secondary battery, an appropriate electrolyte is added at an appropriate concentration range. For example, in an electrolytic solution of the lithium ion secondary battery, a lithium salt such as LiClO4, LiAsF6, LiPF6, LiBF4, CF3SO3Li, and (CF3SO2)2NLi is commonly added as an electrolyte, and the concentration of the lithium salt in the electrolytic solution is commonly set at about 1 mol/L.
As a matter of fact, Patent Literature 1 discloses a lithium ion secondary battery using an electrolytic solution containing LiPF6 at a concentration of 1 mol/L. Furthermore, Patent Literature 2 discloses a lithium ion secondary battery using an electrolytic solution containing (CF3SO2)2NLi at a concentration of 1 mol/L.
Regarding elution of Al, for example, in a lithium ion secondary battery using LiPF6 as an electrolyte, LiPF6 is hydrolyzed to generate HF, and the HF is speculated to corrode Al of the current collector. In addition, Non-Patent Literature 1 and Non-Patent Literature 2 disclose that Al elutes from a positive electrode current collector also in a lithium ion secondary battery using an imide salt based electrolyte, such as, (CF3SO2)2NLi (LiTFSA) and (FSO2)2NLi (LiFSA), having higher hydrolysis resistance than LiPF6.